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The more options you give - the wider the appeal of your game.

Also it can appeal to different age groups - so that a younger or inexperienced player might be able to pass on an easier level.  They might play it through on "easy" and then try it again on a "harder" setting.

There again people also have different equipment - and a person with a non-gaming mouse / keyboard will not have the same experience as someone with it.

Very true...  I never even considered equipment differences, thanks for your input!